1. Field of Invention
The invention relates to an optical encoder and, in particular, to one having a light sensing element with light-sensing cells arranged in a matrix, thereby increasing its precision and reliability.
2. Related Art
When detecting the location of a rotating device (e.g. a motor or a machine axis) or a high-speed moving device, one usually generates a binary identification code in response to each location of the detected device by the on and off of a detecting element in an optical or magnetic ways. For example, several typical optical encoders taught in the U.S. Pat. Nos. 4,451,731, 4,691,101, 4,952,799, and 5,317,149.
These encoders mainly include: a light source for emitting light, a code strip for modulating the light in response to the operation of the rotating device, such as a code wheel, an optical grating disk, or an optical scale, and a photo detector for receiving and detecting the modulated light beam. In generally, some code strips have opaque and transparent regions which are staggered. In this case, the light source and the photo detector are located on opposite sides of a measure element, i.e. the code strip. In other case, the code strips have some reflective regions, and the light source and the photo detector are located on the same side of a measure element.
Refer to FIG. 1, showing the structure of a conventional optical encoder. It is mainly comprised of a main optical grating disk 110, an auxiliary optical grating disk 120, an LED illuminator 130, a photo receiver 140, and a main axis 150. The main optical grating disk 110 is sited on the main axis 150 and driven by the main axis 150 in response to a rotating device. The main optical grating disk 110 has transparent regions 112 and opaque regions 114 which are staggered, as shown in FIG. 2. The light emitted by the LED illuminator 130 illuminates the main optical grating disk 110. Part of the light penetrates through the transparent regions 112 and reaches the photo receiver 140 via the auxiliary optical grating disk 120, while the other part of the light is blocked by the opaque regions 114. Therefore, the transparent regions 112 and opaque regions 114 which are staggered on the main optical grating disk 110 provide a basis for the photo receiver 140 to generate the binary identification code, thereby determining the location of the rotating device. However, since the light source and the photo receiver of the optical encoder are on opposite sides of the grating respectively, only one side can be used to generate codes. The resolution is thus limited and the device cannot be made too thin.
The structure of another conventional optical encoder is shown in FIG. 3. It includes: a code wheel 210, an LED illuminator 220, and a photo receiver 230. The code wheel 210 is driven by a wheel (not shown) in response to a rotating device. Moreover, the code wheel 210 has reflective regions 212 and non-reflective regions 214 which are staggered. The LED illuminator 220 illuminates the reflective regions 212 on the code wheel 210, and then the photo receiver 230 disposed on the same side as the LED illuminator 220 receives the modulated light beam directly reflected from the reflective regions 212 to obtain a binary identification code indicating the location of the code wheel 210. The location of the rotating device is thus determined for subsequent controls of the speed and stroke of the rotating device.
In the conventional optical encoder, better control precision is usually achieved by increasing its resolution. The increase of the resolution is often achieved by changing the number of transparent and opaque regions on the code strip (or the number of reflective and non-reflective regions) or by adopting several code strips and several photo detectors. However, this is likely to increase the thickness of the optical encoder, contrary to the trend of miniaturization. Moreover, errors occur when the code wheel is dirty. Therefore, the existing optical encoders need to be improved.